1. Field of the Invention
Embodiments of the present invention generally relate to a compressed air energy system having variable generation modes.
2. Description of the Related Art
Compressed air energy storage (CAES) is a historically proven method of “storing” electric energy for later supply to the bulk power grid. The first utility scale CAES facility began service in 1978, in Huntorff, Germany, with a second plant in McIntosh, Ala., going into service in 1991. CAES plants compress air with an electric motor-driven compressor, injecting the air at high pressure into an underground storage cavern. Subsequently, when power is needed for the grid, high-pressure air from the cavern is routed through one or more expansion turbines, performing work and driving an electric generator, producing power for the grid.
In these initial applications, CAES was implemented to produce cost-effective peak power by shifting low-cost, off-peak energy into high demand hours. One distinctive feature of these initial CAES installations was the use of a single electrical machine to function as a motor to drive the compression train, and as a generator when withdrawing air from the cavern. This was accomplished by linking the “motor/generator” via clutches to a compressor train and to the expanders. One clutch would always be engaged, with the other disengaged, allowing either function to be performed by this single electrical machine. The advantage of this design is the avoidance of an additional large electrical machine (i.e., a motor or a generator).